Glycidol is a versatile intermediate for further reaction, and has been known since the 19th century.
However, the large scale use of glycidol has not been fully realized in the past due to its poor availability in required quantities and consequent higher cost for downstream applications. This invention concerns a new route to this potentially valuable intermediate, produced preferably in a continuous manner, in high quality and derived particularly from renewable resources, such as glycerine. In addition the proposed process also provides substantially pure salt or pure brine as side product, in high quality for commercial use. The glycidol produced is particularly of high quality and this enables conversion to better quality, more useful, derivatives, e.g. thermosets of polyols.
The most common method for production of glycidol involves the oxidation of allyl alcohol, which itself is prepared from mainly propylene oxide by an isomerisation process. The propylene oxide is prepared from propylene and an oxidation agent such as hydrogen peroxide, percarboxylic acid, hydroperoxides etc., catalyzed by various types of catalyst. The known disadvantages of this method are in the numerous reaction steps for preparation of allyl alcohol prior to its oxidation, the extraction of glycidol from obtained homogeneous aqueous reaction mixture containing unreacted allyl alcohol and by-products such as glycerol, acrolein, β-hydroxy propionic aldehyde, glycerol allyl ether and decomposition products of catalyst, and the purification of glycidol after its isolation from the aqueous solution. Moreover, the catalyst used, such as tungsten trioxide, decomposes during the oxidation and contributes to higher production costs.
The previous procedures moreover involved precursors derived from fossil oil.
Other approaches have been described using glycerine as the starting precursor. These approaches are now of growing interest, as glycerine is an important bio-derived by-product of the manufacture process for bio-diesel, itself an important development in the growing green-tech industry for, e.g. new transportation vehicles. Glycerine derived from this new route is available in feedstock quantities, however with a new profile of associated impurities as a consequence of its bio-diesel refinery preparation route. The impurities can be fatty acids, protein, and/or various ionic salts, the concentration of which in the glycerol varies according to the source of the biomass used.
Previous routes to glycerine came from fossil oil, via the propylene route, and thus, this invention preferably concerns the use of bio-derived glycerine with its consequent impurities, to prepare certain grades of glycidol in a cost effective, preferably continuous process. However, should other sources of glycerine become available in the future, the process would be still applicable.
One approach of the conversion of glycerine involves carbonation to glycerol carbonate, which product subsequently forms glycidol and CO2, by decarboxylation of the carbonate. In the early 1950's, a new process for preparation of glycidol was developed involving the two-step synthesis by carbonylation of glycerol via glycerol carbonate. In the first step, a cyclic carbonate is transesterified with the glycerol in a solvent comprising an organic carbonate or mixture of carbonates, in the presence of a solid catalyst to form cyclic glycerol carbonate. The glycerol carbonate can also be prepared by carbamoylation of glycerol with urea, by oxidative carbonylation of glycerol with the mixture of carbon dioxide and oxygen, or by reaction of glycerol with phosgene under mild conditions (60 to 130° C., in the presence of a catalyst). The second step, i.e. thermic decarboxylation of glycerol carbonate is technologically more difficult. The reaction is usually kept at the temperature at 155 to 240° C. under vacuum to give glycidol and carbon dioxide, the yield of glycidol being about 60%, or with a basic catalyst. The reported yield of glycidol in the latter is over 80%, based on carbonate. Direct production of glycerol carbonate from glycerol and carbon dioxide under supercritical conditions or in the presence of tin or cerium catalysts has also been reported.
Glycerol carbonate is a relatively new material in the chemical industry, but one that could offer some interesting opportunities, as it can be prepared directly and in a high yield from glycerol. The advantage of the process for preparation of glycidol via glycerol carbonate is relatively simple two-step process. However, the lower yield of glycidol in the second step is considered to be the substantial disadvantage and attention has recently been paid to solve it. Therefore there is need to consider other routes.
Another route involves conversion of acrolein made from glycerol: WO2012/003519. In yet another approach, glycerine can be hydrochlorinated to monochloropropanediol, MCH, specifically the isomers 3-chloro-1,2-propandiol or 2-chloro-1,3-propandiol, using hydrogen chloride gas or aqueous hydrochloric acid. The MCH then can be converted to glycidol using alkaline agents yielding a reaction product mixture of the glycidol, corresponding salt and water (see, for example U.S. Pat. No. 2,070,990, U.S. Pat. No. 2,224,849, DE 1041488, DE 1226554, U.S. Pat. No. 3,457,282, U.S. Pat. No. 5,965,753, GB 822686, U.S. Pat. No. 5,198,117, U.S. Pat. No. 4,105,580, or U.S. Pat. No. 6,156,941). Historically, MCH was practically the first and only starting material for an industrial-scale preparation of glycidol. In fact, both MCH, as its isomers 3-chloro-1,2-propandiol or 2-chloro-1,3-propandiol, and glycidol itself were intermediates in the course of a multistage and expensive production of glycerol from allyl chloride via epichlorohydrin.
In order to purify the epoxy compound, azeotropic distillation has been employed to remove water (see, e.g. U.S. Pat. No. 2,248,635, U.S. Pat. No. 3,247,227, RU 2130452).
The modern process for production of glycidol starting from MCH is seen as a suitable technological alternative again. MCH can be now advantageously prepared by the recently developed technology of catalysed hydrochlorination of glycerine, as described in WO2005/021476, or WO2009/016149.
WO2009/016149 can be deemed as the closest prior art, hereafter cited as '149. The invention in '149 relates to a process for manufacturing glycidol comprising at least the following steps: a) glycerol and a chlorinating agent are reacted to form monochloropropanediol in a first reaction medium, following a preferred process described in WO2005/054167; and b) at least one basic compound is reacted with at least one part of the first reaction medium from step a) to form glycidol and a salt in a second reaction medium, the organic component of which has a monochloropropanediol content before reaction with the basic compound greater than 100 g/kg of organic component, following the processes described in FR 07/153375.
The object in '149 further is to obtain glycidol from MCH, at least one part of which was prepared by the reaction between glycerol and a chlorinating agent.
The MCH as a starting material for glycidol synthesis may be isolated from reaction mixture, or the reaction mixture was used without purification. No particular reference is made that the starting with MCH which has very reduced impurities can yield useful production of glycidol. Low levels of ester or acid in the MCH are not described.
The reaction mixture in '149 after saponification was than treated to isolate glycidol, solvent and other organic components and to obtain purified glycidol-based product in the first step, while water and salt were isolated in the second step, or the water-based composition was recycled to an electrolysis process.
The art '149 further focuses on use of solvents to separate the glycidol from the reaction mixture. It is notable that amongst solvents quoted in '149, as suitable for use in the separation treatment of the formed glycidol and salt mixture, are such solvents as described below taken from the passage in '149:
The extraction solvent is generally such as described in Application FR 07/55697 by Solvay SA, of which the content, and more specifically the passage from page 10, line 23 to page 13, line 12, is incorporated here by reference.
‘The extraction solvent is generally an organic solvent which may be chosen from epoxides other than glycidol, esters, ketones, ethers, alcohols, carboxylic acids, organic phosphates and phosphine oxides. The organic solvents may contain water, preferably up to saturation. Dichloropropanol, epichlorohydrin and mixtures thereof are particularly preferred extraction solvents.’
In order to minimise these multi issues and obtain useful yields of glycidol with limited impurities, and to obtain a substantially pure salt as a valuable by-product, we have achieved a process which can be optimised readily around a number of parameters.
Thus, the risk factors that can affect the yield of glycidol are many and some are described in the following:                easy dimerization and rearrangement of glycidol,        presence of water and too high temperature of epoxidation or presence of free basic- or acid-acting agents even at relatively low reaction temperature can increase the risk of polymerization, condensation or hydrolysis, possibly with the presence of the rest of salt in relatively low concentrations,        above mentioned factors can be further complicated by serial reactions if there is prolonged residence time in process reactor,        loss of glycidol during distillation by entraining to the solvent and water,        low concentration of by-products is usually obtained during the saponification in diluted solutions; however, the higher the concentration of glycidol in the solution, higher is the content of by-products, which brings a significant problem in a continuous arrangement of reaction when carried out, e.g. in a CSTR reactor.        at low reaction temperature, e.g. below 5° C., the rate of dehydrohalogenation is low and longer retention time of reaction is necessary, especially in case of 2-chloro-1,3-propandiol isomer,        when using unrefined or poorly refined raw material (i.e. technical reaction mixtures) of the MCH, or indeed the starting glycerol from which MCH is derived, many impurities can become introduced into the reaction mixture and this causes problems with their removal, both from the glycidol and from the salt or brine,        the reaction mixture containing glycidol and traces of salt is unstable and is useful only as an intermediate for immediate processing, because despite rapid neutralization, the content of glycidol can drop significantly, even at the low temperature, e.g. below 0° C.,        in particular, actual kinetic trials show that during the addition of the sodium hydroxide solution, when the alkalinity of reaction mixture rises rapidly when system approaches the stoichiometric conditions, the mixture swells prior to reaching the equivalence point of—sodium hydroxide and MCH; this is connected with the formation of glycidyl ethers, glycerine a and polyglycerine, and (in case of processing in some alcohols) with formation of alkyl ethers of glycerol,        subsequently the formation of undesired by-products is accelerated with the molar excess of sodium hydroxide to monochlorohydrin, especially in combination with elevated reaction temperature, which can also lead to spontaneous polymerization.        extraction processes, as e.g. described in '149, can be used to isolate the glycidol from the reaction mixtures, instead of, for example distillation, in order to prevent the contact of the epoxide group with the sodium ions at elevated temperature; the disadvantage of such extraction processes is that there is the low efficiency of the separation in one step and the necessity of the multistage process for collecting a substantial portion of the glycidol. A part of organic compounds still remains in an aqueous phase containing ionic species, e.g. NaCl, and this makes the next processing impossible without an additional technological step. Thus the major problem in the extraction process is that the organic glycidol-rich phase still contains sodium ions which catalyse the degradation of the glycidol.        
The next bottleneck is that the water-rich phase, which also contains ionic species, e.g. NaCl, still contains residual MCH and the rest of the glycidol, which must be recovered by further multistage extraction processes.